Question about Background images and Glass during rendering

I am rendering an old car. I added a background image to the Render. It seems that where I have glass in the car, the only thing I see behind the glass is the background image and not other parts of the car that should be seen behind the glass. For example, if the car is sitting at an angle, there should be parts of the car behind the windshield being displayed along with parts of the background image. In my Render, the background image is displayed, but the parts of the car that would normally be seen behind the windshield and in front of the image are not displayed. Can someone explain how to fix this?

It sounds like something I've done myself before and it was because I had forgotten to check the transparency box and had an alpha value that was below 1, making it see through.

I have the vague feeling there was something else that causes that sort of thing, but I cannot remember what it was. If checking the transparency box doesn't solve your problem, I can try to look for it...whatever it was...